A virtual machine (VM) does not keep time exactly the same way that a physical machine does. When running low latency, high throughput applications like ESP, these differences can cause timekeeping inaccuracies and other problems.
There are configuration options that can mitigate these problems: such as configuring the VM to a particular set of CPUs, using locked memory (no sharing between VMs). But they involve trade-offs with the reasons, such as better resource utilization, for using VMs in the first place.
For a detailed discussion of timekeeping in VMware virtual machines, refer to http://www.vmware.com/files/pdf/Timekeeping-In-VirtualMachines.pdf.
If you want to run ESP on a VM, we highly recommend that you engage the vendor for help in configuring the VM for use with ESP.